1. Field of the Invention
The present invention relates to brake control for controlling a braking force applied to a wheel of a vehicle.
2. Description of the Related Art
A brake apparatus that generates hydraulic pressure corresponding to an operation of a brake pedal in a hydraulic circuit and applies a braking force to a wheel by supplying the hydraulic pressure to a wheel cylinder is conventionally available. A plurality of electromagnetic valves are provided in the hydraulic circuit, and by open/close controlling each electromagnetic valve so as to adjust supply/discharge amounts of a working fluid to/from the wheel cylinder, the hydraulic pressure is controlled such that an appropriate braking force is applied to each wheel.
This type of electromagnetic valve includes an electromagnetic valve that is maintained in a driven state by applying a steady-state current, which is lower than an activation current, thereto steadily following application of the activation current so as to achieve an improvement in controllability and a reduction in power consumption. This characteristic is seen in switch valves such as a normally open master cylinder cut valve for connecting or disconnecting a master cylinder and a wheel cylinder, a normally closed simulator cut valve for connecting or disconnecting the master cylinder and a stroke simulator, and a normally closed isolation valve for connecting or disconnecting a front wheel side hydraulic passage and a rear wheel side hydraulic passage, for example (see Japanese Patent Application Publication No. 2006-17181 (JP-A-2006-17181), for example).
For example, a normally closed switch valve is opened by applying an activation current thereto when a vehicle is driven, and then maintained in a predetermined open state by maintaining a constant steady-state current. This is achieved using hysteresis in the opening characteristic of the switch valve, which enables the switch valve to be held in an open state at a lower current value once the switch valve has been opened. By maintaining the open state in this manner, the required hydraulic pressure can be supplied as soon as a brake pedal is depressed, and as a result, control responsiveness can be maintained at a favorable level. Meanwhile, since the switch valve is held at a lower steady-state current than the activation current, a reduction in power consumption can be achieved in comparison with a case in which the activation current is maintained at all times.
However, this type of switch valve uses a power supply source that is shared by other control applications. Therefore, the valve may close due to a voltage reduction occurring when the electrical load temporarily increases, for example when an air-conditioning apparatus is activated or headlights are switched ON. When the switch valve temporarily closes in this manner, it may be impossible to open the valve using the valve opening characteristic described above by applying the steady-state current alone. In a case where a simulator cut valve is closed in this manner, for example, a stroke simulator cannot function sufficiently well when the driver depresses the brake pedal, and as a result, the pedal feeling deteriorates. Further, when an isolation valve is closed in this manner in a case where vehicle travel control is performed automatically, for example, the required working fluid is not supplied to the respective wheels, leading to deterioration of the control performance. With respect to a normally open switch valve, on the other hand, the valve is operated in a closing direction by the activation current and maintained in a closed state by the subsequent steady-state current, but when the valve opens due to a temporary voltage reduction, as described above, similar problems may occur.